Evaluation Methods
Many optical properties can be determined in different ways.
For instance, the
critical angle can be
found in two ways, namely:
- where the refracted angle is 90°
- where the reflected light intensity is greater than 0
The
Brewster angle can be
found in three ways, namely:
- where the refracted angle is perpendicular to the
reflected angle
- where the reflected light intensity is a minimum
- where the reflected light is totally polarized
Furthermore, many properties are related to others. In particular, both
the critical angle and the Brewster angle are related to the
index of refraction.
In this lab,
you are going to
design an
experiment to measure one of those angles.
You're then going to test it.
After that you will refine it
or try another method of
determining it.
You'll then do the same thing for the other angle.
Finally, you'll compare the values from the index of refraction
given by the two (or more) methods.
If you don't think you can improve on one or both of your methods,
you can also measure the normal incidence reflectivity
of the material since it is also related to the index of
refraction.
Week 1: Introduction
In the lab: Optical effects exercise
For next week:
Look up each term in question,
(i.e. critical angle, Brewster angle, normal incidence reflectivity),
and write a description of what it means
and a couple of sentences indicating the general principle for measuring
it.
In the lab you will attempt to find equipment you can use for the
purpose and try it out.
Week 2: Experiment prep
Pick one of the angles, and examine the equipment in the lab to
come up with an experiment for measuring it one way.
For next week:
Write up a procedure for doing the experiment and bring it to the lab
with you.
Week 3: Perform experiment
Perform the experiment you devised.
Be sure to figure out a way to use multiple trials.
For next week:
Write up a lab report for the experiment and bring it to
the lab
with you.
Tips for the lab report:
-
Write the "Methods" section in the past tense, not in point form. You
are describing what you did, not what the reader should do.
-
Make sure to include at least one diagram. Would you be able to
picture someone else's setup from a text description alone?
-
Show sample calculations in your "Results" since it's often not
obvious how to get from your data to your calculated results and
uncertainties.
-
In your "Discussion", if you suggest a possible source of error,
place bounds on the size of effect it could have.
-
Make sure you actually state the value you got for the thing you set
out to measure with its uncertainty in your "Conclusions".
-
Hand in one report for each person in a group.
-
When you revise an experiment, you will attach the original
report along with the revised one.
Week 4: Experiment prep
Hand in lab report from last week.
For
the other angle, examine the
equipment
in the lab to
come up with an experiment for measuring it one way.
For next week:
Write up a procedure for doing the experiment and bring it to the lab
with you.
Week 5: Perform experiment
Perform the experiment you devised for measuring it.
For next week:
Write up a lab report for the experiment and bring it to
the lab
with you.
Week 6: Revised experiment prep
Hand in lab report from last week.
Figure out how to
improve the first experiment you did.
This can be either by changing the way you did the original experiment,
or by devising a new experiment to measure it one of the other
ways.
Some options for improving/changing the first experiment:
-
If you didn't actually determine a value for the angle you
were finding, make sure you do that. Also make sure to determine the
index of refraction from that.
-
If you only did a single trial, do more.
-
If you only used a single light source, try using another.
-
If you used a single sample, repeat for a different sample (or
samples) to see if you can measure a difference.
-
If there is more than one way to identify when you have found
a specific angle, and you used only one, then use the other (or an
other) way.
-
If there is only one way to identify when you have found
a specific angle, you can do measure the index of refraction
directly using Snell's Law for a range of angles.
-
When you revise an experiment, you will attach the original
report along with the revised one.
For next week:
Write up a procedure for doing the experiment and bring it to the lab
with you.
Week 7: Perform experiment
Perform the experiment you devised for measuring it.
For next week:
Write up a lab report for the experiment and bring it to
the lab
with you. Be sure to discuss whether/how the improvements you
proposed worked.
Some tips for the lab report:
-
Remember to attach the first lab report to your new one.
-
Since your original report will be attached, you can draw attention
to what changes you made to the original experiment.
-
In your Introduction,
Methods, and Results, if there was
anything not explained or poorly explained in the original, you can
clarify.
-
In your Discussion, you can address whether and how your
revised experiment improved on the original.
-
In your Conclusions, you can compare your results for
the revised experiment to those in the original.
Week 8: Revised experiment prep
Hand in lab report from last week.
Figure out how to
improve the second experiment you did.
This can be either by changing the way you did the original experiment,
or by devising a new experiment to measure it one of the other
ways.
Some options for improving/changing the second experiment:
-
If you didn't actually determine a value for the angle you
were finding, make sure you do that. Also make sure to determine the
index of refraction from that.
-
If you only did a single trial, do more.
-
If you only used a single light source, try using another.
-
If you used a single sample, repeat for a different sample (or
samples) to see if you can measure a difference.
-
If there is more than one way to identify when you have found
a specific angle, and you used only one, then use the other (or an
other) way.
-
If there is only one way to identify when you have found
a specific angle, you can do measure the index of refraction
directly using Snell's Law for a range of angles.
-
When you revise the experiment, you will attach the original
report along with the revised one.
Alternative: If your second experiment worked out well, and you don't
need to revise it, you can explore something else.
For next week:
Write up a procedure for doing the experiment and bring it to the lab
with you.
Week 9: Perform experiment
Perform the experiment you devised for measuring it.
For next week:
Write up a lab report for the experiment and bring it to
the lab
with you. Be sure to discuss whether/how the improvements you
proposed worked.
If you want, you can go ahead and complete the requirements for
next week as well.
Week 10: Compare and combine results
Write up a comprehensive lab report that calculates that index of
refraction from each of the experiments,
(Brewster angle and critical angle),
and compares the values.
Many sections can be mostly cut-and-paste from your previous
reports.
- Introduction can have one part for each.
- Methods can have one part for each.
- Results can have one part for each.
- Discusssion will be different. It can start with a section on
sources of uncertainty common to both experiments. After that,
it needs to have a section about comparing the two experiments
as methods of determining the index of refraction.
Discuss whether either of the experiments stood out as a better way to
determine the index of refraction. For instance:
- Was one method easier to perform?
- Did one method give more consistent results?
- If you used different materials in the different experiments,
did the index values from both fall within expectations?
- Conclusions should give you best value for the index of refraction,
based on the discussion above, and indication of whether or not it
agreed with
expectations.
Week 11: Submit final report
Hand in report from last week.
Week 12
Get marked report from last week.
